Highly efficient particulate removal from hot gas streams is important in various process applications including, for example:
advanced Combined Cycle Systems where High Temperature/High Pressure (HTHP) combustion gases must be efficiently cleaned before they are introduced into gas turbines;
gasifiers where HTHP process gases must be efficiently cleaned before further processing;
Hydrocarbon processing where gas streams must be cleaned, and catalysts removed from the off gases and recovered; and
waste incineration processes where high quality clean-up of emissions is required.
Current HTHP cleaning technology has a number of deficiencies rendering, e.g. an effective adaptation of Advanced Combined Cycle Systems (incorporating steam as well as gas turbines) for power generation expensive, complicated and unreliable. It has been suggested to utilize two stages of refractory lined cyclones in pressure vessels to remove dust from the exhaust gases, in order to minimize the dust content to a level sufficiently low to preclude erosion of the gas turbine. Very fine dust escapes separation in cyclones, however, and flows with the exhaust gases through the gas turbine. Final clean-up of the combustion gases to regulatory particulate limits (for emissions) is accomplished by conventional electrostatic precipitation in a non-pressurized location downstream of the gas turbine. This cleaning system is not completely satisfactory as the gas entering the gas turbine may still contain abrasive particles which may cause damage to turbine components. The system is further rather complicated and space consuming, and leads to larger and more costly pressure vessel constructions.
It has also been suggested to use HTHP Ceramic Filters in pressure vessels for cleaning of hot exhaust gases. See, for example, commonly assigned U.S. Pat. No. 4,869,207 and commonly assigned application Ser. No. 07/574,550, now allowed. The above identified patent and patent application are related to filtration housings with candle type or Asahi type porous ceramic filter tubes supported vertically by horizontal cooled or non-cooled support plates. The size of the filtration housing is limited when using this type of arrangement, i.e., the diameter of the tubes cannot be increased beyond about 2-4 m. The tubes are preferably also fixedly supported at both ends which can cause problems with the sealing of the tubes to the support plates. Temperature and expansion differentials may also cause difficulties.
When using candle type filter tubes, the solids are separated on the outside of the filter tubes, whereas with Asahi type tubes, the solids are separated on the inside of the tubes. Both types are periodically cleaned by high pressure reverse air or gas pulses. A major limitation of the present HTHP filter technology, however, is the scale-up of the units to larger capacities.
The candle type filter housing units are built as refractory lined vessels, and the support plates are made of steel or castable refractories. The size of the filter housing is presently limited by construction considerations, i.e., the practical limit for the diameter of a pressure vessel with candle type filters is about 2-3 m. It is therefore not possible to scale up the filter unit and increase the filtration area simply by adding additional filters, as an increase in the number of filter tubes would require a scale up of the pressure vessel itself due to the increase of the required support plate area. The filtration velocity is presently limited to about 10 cm/s.
In an Asahi type filter, in which filter tubes in a refractory lined pressure vessel normally are vertically supported by a cooled support plate, the diameter of the filter housing is also restricted by construction factors to a max of 2-4 m. It is difficult to build large water cooled support plates having a diameter &gt;2 m, due to expansion of the refractory lined vessel and due to the required rigidity of the support plate.
Filtration housings themselves have been previously made with refractory lined, non-cooled walls. There may have been several reasons for not cooling the walls of the housings:
there has been little practical knowledge of water cooled walls in pressurized surroundings;
there may have been a concern for water getting into the HTHP system; and
the cooling of walls of a housing inside a pressure vessel may have been considered too complex.
Filtration housings have also been made as separate pressurized vessels, with the inside of the housing insulated. The insulation, however, has had to be very thick, e.g., 300 mm or so. A water cooled filtration housing could not be pressurized, however, as a separate pressure vessel since it would not survive the high pressure.
The low filtration area per filter housing volume is also a drawback of the present HTHP filters. The filter units are relatively small, corresponding to a max 40 megawatt (MW) power plant. Scale up of a power plant requires an increased number of filter housings or filter units, and consequently, for normal size power plants of 300 MW, at least 8 filter units are required. Utility power plants having a size range 100-500 MW will always need an increased number of filter units.
The combination of two or more filter units in a stacked or side-by-side arrangement leads to a very space consuming design and also complicates the configuration of HTHP ductwork to and from the filtration housings, both for gas and solid streams. As a result, the system is very costly and susceptible to failures and other problems. The stacking of several filtration chambers also leads to expansion problems in the combined construction.
Arranging several smaller filtration chambers beside each other in a pressure vessel would also be costly. The size of a pressure vessel pressurized to &gt;5 bar is very crucial to the total cost of the system, and therefore the pressure vessel should be kept as small as possible, and the equipment inside the vessel should be arranged in as compact an arrangement as possible.
It is known from other technological areas to use horizontal filters open only at one end, as shown in U.S. Pat. No. 4,468,240 by Margraf or U.S. Pat. No. 4,046,526 by Phillippi. These filters are not related to high temperature or high pressure filtration units, however, and do not have cooled filtration housing walls.
It is an object of the present invention, therefore, to provide a HTHP ceramic filtration unit for pressurized combustion, gasification or related processes to overcome the aforementioned problems and to furnish an inexpensive, simple and reliable HTHP clean-up system.